A math professor mentioned that the final segment of Fermat's name would probably have been pronounced [t] because of "where he was from." She didn't clarify further but I looked up where he's from and he was Gascon and a speaker of that language (sometimes considered a dialect of Occitan). Could someone confirm that this would have been the case in the Occitan of his day? I searched around and couldn't find anything attesting to this.
Fermat was fluent in multiple languages, including French and Occitan. Though he was born and raised in Beaumont-de-Lomagne (Occitania), his paternal family was originally from Catalonia:
In the second half of the fifteenth century, the Fermat family apparently emigrated from Catalonia to Beaumont-de-Lomagne...
- Pierre de Fermat (1601?-1665): His life besides mathematics, European Mathematical Society Newsletter (No.42)
As such his name's authentic pronunciation was likely
A contemporary prestige dialect French pronunciation would likely have been
/fer'ma(t)/, since texts from the late 17th century imply that word terminal -t should only regularly be pronounced when occurring before a vowel.1
1. L'art de prononcer parfaitement la langue françoise, 1689 (p.349)